Method of handing off traffic in a low orbit satellite communication system serving terminals and communication system using a method of this kind

ABSTRACT

In a method of handing off traffic in a low orbit satellite communication system serving terminals, a terminal being connected to a terrestrial communication network by means of a connection station, handing off of the satellite-terminal and satellite-connection station links is possible only within a particular area. The decision to hand off the traffic is based on deterioration of the quality of service and of the radio environment for a group of terminals of said area. The system for implementing this method includes transmit/receive terminals and terrestrial connection stations.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention concerns a method of handing off traffic in a low orbitsatellite communication system serving terminals which may or may not bemobile and which are transmitter-receivers or receivers only, a terminalbeing connected to a terrestrial communication network by means of aconnection station.

The invention also concerns a communication system using a method ofthis kind.

2. Description of the prior art

Satellite-based communication with mobile terminals has until now usedtwo types of orbit: geostationary satellite orbits and highly inclinedelliptical orbits, both kinds being situated, on average, above areasknown as the "Van Allen belts" characterized by a high concentration ofparticles. More recently lower orbits have been envisaged. Theiraltitude is between 800 and 2,000 km. One feature of satellitecommunication systems using these orbits is the possibility ofcommunicating with a large number of mobile terminals, portableterminals, for example. The difference between orbits at altitudes abovethe "Van Allen belt" and those at lower altitudes is that the closer tothe earth the satellite is located the lower is the attenuation.

A CCIR report (document US IWP 8/14-52, 1 Aug. 1990) entitled "Technicalcharacteristics of a personal communication mobile satellite system"describes a low orbit satellite communication system using multibeamantennas. The resulting constellation of satellites comprises 77satellites to provide global coverage. It is made up of seven planeseach of eleven satellites each in a circular polar orbit. However, tomaintain a call when a terminal leaves the coverage area of a firstsatellite, a system of this kind includes intersatellite links. Asolution of this kind requires demodulation on board the firstsatellite, routing of the information, transmission to a secondsatellite and relay to the connection station. This is very complex andcostly.

An object of the present invention is to minimize the cost of the spacesegment by using transparent satellites and eliminating allintersatellite links.

SUMMARY OF THE INVENTION

In accordance with the present invention handing off of thesatellite-terminal and satellite-connection station links is possibleonly within a particular area; the decision to hand off the trafficbeing based on deterioration of the quality of service and of the radioenvironment for a group of terminals of said area.

In a first variant the traffic is handed off from a first satellite to asecond satellite (14) having sufficient elevation over a particular areawhich is a satellite cell.

In a second variant the traffic is handed off from a first beam to asecond beam of the same satellite.

In a first embodiment the traffic is handed off from the first beam tothe second beam progressively; all new calls being set up via the samesatellite; the decision to hand the remainder of the traffic off to thesecond beam being based on deterioration of the quality of serviceprovided in the first beam and the radio environment of a group ofterminals.

In a second embodiment the traffic is handed off from the first beam tothe second beam globally; the instruction to hand off from the firstbeam to the second beam being sent to the connection station to all theterminals of the respective area when all parameters of the radioenvironment of said terminals reach a particular limit.

A particularly advantageous satellite communication system employing themethod of the invention uses two constellations WALKER (1 389 km, 47°,24/08/3) and WALKER (1 389 km, 55°, 24/08/3). A system of this kind is aworld coverage system which complements existing communication systems.It enables a great variety of services to be offered. However, its mainfeature is that it can adapt to traffic and service situations differingfrom region to region.

Market analysis has shown that the main requirement for communication isregional, with niche markets of a worldwide nature. The architecture ofa system of this kind is designed to adapt to this situation.

These applications concern closed networks and public networks. Thebasic service is digital telephony with all the ensuing dataapplications. Other services can also be provided using the Dopplershift due to the movement of the satellite. These are radio-navigationor radio-location services. There is also provision for using theradio-location service to carry out some base station assignmentfunctions.

The invention will now be described in more detail with reference to theappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the traffic handing off method in a prior art satellitesystem.

FIGS. 2 through 6 show the traffic handing off method in accordance withthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The satellite system or "constellation" is a worldwide coverage systemwhich complements existing terrestrial communication systems. In thissystem a terminal, which may be mobile or not and is atransmitter-receiver or a receiver only, is identified, located andlinked by a satellite of the constellation through a connection stationto the fixed public network enabling the user to access all services ofthe public switched telephone network or the future ISDN.

The radio frequency resources allocated to the set of terminals aredivided into satellite cells which each represent an area of smallradius as compared with the satellite coverage area but of large radiusas compared with a terrestrial cell of an earth cellular network of the"Groupe Special Mobiles" (GSM) type, for example. This cell is connectedto a connection station and the terminals which are within thisgeographical area are connected to this station. The system must beaware in real time of all terminals that are "active" (calls set up orbeing set up) and terminals that are "on standby"; apart from when it iscommunicating, a terminal is regarded as on standby or inactive. In thissystem the terminals therefore belong to a satellite cell connected to aconnection station which provides the connection to the earth cellularnetwork.

In the "standby" mode the terminals receive a signal on a specialfrequency identifying the cell in which they are located and enablingthem to determine the parameters assigned to that cell. Any new terminalbecoming active in this cell sends its identification code. Theprocedure for updating the terminal location database is identical tothat used in the terrestrial mobile radio communication infrastructure.

Compared with cellular networks, the satellite system does not require alarge number of exchanges of data between the terminal locationdatabases and the connection station. This is because of:

The large coverage radius of the satellite cell.

The fact that in the satellite cell network the handing off of aterminal from one cell to another is regarded as a very infrequentoccurrence.

As far as a terminal is concerned, the network access coordinates arethe radio frequency resource data and a channel frequency band. As faras the connection station is concerned, in addition to the data specificto the terminal, a satellite number is assigned in order to provide thelink with the terminals in the cell associated with the connectionstation.

In a given cell the location and logging of terminals and callprocessing are exactly the same as in the terrestrial cellular network.Where call processing is concerned, the connection station processes anadditional item of data comprising the number of the satellite havingthe best elevation for the cell in question. One of the connectionstations includes additional equipment needed to track the satellitesproviding the radio coverage of the cell. These measurements transmittedto the ground control segment are used to calculate the ephemerides ofthe satellites and to deduce therefrom connection station antennapointing tables.

In a cell the connection of the terminal to the terrestrial network istherefore accomplished by means of a connection station. This stationhas the following functions:

It manages a set of pre-assigned radio frequency resources and a set ofoverflow resources.

It handles the setting up and clearing down of links with the terminals.

It handles traffic for the satellite having the best elevation. Thisinvolves tracking the satellites and handing the traffic off from onesatellite to another according to the respective elevation of eachsatellite.

It provides the connection to a terrestrial central office.

To implement these functions the connection station is made up of twoseparate entities:

a radio communication unit comprising the antenna and radio frequency(RF) subsystems and a signal processor subsystem (modems, etc),

a management and connection unit managing the radio frequency part ofthe station (management of allocation of radio frequency resources,preparation and execution of satellite handing off, etc) and thesubsystem managing exchanges with the terrestrial transmission segment.

Each connection station has three antennas with their associated RFsubsystem. Two station antennas track the satellites. The third antennaserves as a back-up for the other two. Each antenna has a trackingsystem programmable on the satellite ephemerides. These pointing tablesare supplied at regular intervals by the satellite control segment ofthe system.

From a general point of view, and as compared with a terrestrialcellular network, the system does not introduce any modifications intothe overall architecture of the terrestrial network. The specialfeatures are restricted to the connection station and consist in thechoice of the satellite having the best elevation for the area inquestion and in management of the handing off of traffic from onesatellite to another.

To minimize the cost of the space segment the satellites are transparentand there are no intersatellite links.

The result of these two choices is that to maintain a call themobile-satellite and satellite-connection station links are handed offfrom one satellite to another simultaneously. Because of this mode ofoperation:

location logging (roaming) is based on the connection of a terminal tothe nearest connection station,

there is a limiting distance below which the handing off of thesatellite-mobile and satellite-connection station links is possible.

In estimating this distances allowances must also be made for:

an acceptable elevation in order to guarantee an adequate quality ofservice,

the feasibility of the terminal and the connection station acquiring thecarrier and synchronizing the new channel quickly.

To highlight the advantages of a solution of this kind FIGS. 1, 2 and 3respectively show handing off requiring an intersatellite link andhanding off in accordance with the invention which does not require anysuch link.

In FIG. 1A the coverage area 10 of the first satellite 11 includes aconnection station 12 and a terminal 13.

In FIG. 1B the satellite 11 has moved but the connection station 12 andthe terminal 13 are still in its coverage area.

In FIG. 1C the coverage area 10 of the satellite 11 no longer includesthe connection station 12. The terminal 13-connection station 12 linkmust then pass through a second satellite 14 whose coverage area 15includes said station 12.

In a first embodiment of the method of the invention the first twosituations as shown in FIGS. 2A and 2B are the same. However, in FIG. 2Chanding off from the first satellite (11) to the second satellite (14)has taken place. The terminal 13-connection station 12 link passes onlyvia the second satellite 14.

FIG. 3 shows a second embodiment of the method of the invention. In FIG.3A the coverage area of the first beam 17 includes a connection station12 and a terminal 13. In FIG. 3B the coverage area of the beam 17 nolonger includes the connection station 12 and the terminal 13. Theterminal 13-connection station 12 link must then pass via the beam 16 ofthe same satellite 11 whose coverage area includes said station and saidterminal.

The feasibility of handing off depends on complying with a set ofconstraints concerning the setting up of a link with a new satellite orwith a new beam. The synchronization time-delay, shift and carrierfrequency must be kept within the performance limitations of the signalprocessor units. The relative position of the connection station and thevisible satellites are known from the ephemerides with sufficientaccuracy. Only the position of the terminal relative to the connectionstation is unknown and therefore introduces a degree of uncertainty inrespect of the elevation, synchronization, received carrier frequency.

The remainder of the description considers the first embodiment by wayof example.

Consider the Doppler shift. At any given time a connection station knowsexactly the Doppler shift of the entering satellite and that of theexiting satellite. This station can communicate to the terminal theDoppler shift that it sees at the time of handing off and that theterminal also sees subject to a small degree of uncertainty. In thiscase the terminal is able to pre-correct its receive and transmitfrequency at the time of handing off. The uncertainty in respect of theDoppler shift to be compensated is then reduced to that resulting fromthe uncertainty in respect of the position of the terminal.

For a terminal of a given cell connected to a ground station the radiofrequency resources it is allocated comprise a transmit/receive intervalnumber (combination of TDMA or CDMA access and beam hopping in which Ncoverage spots on the ground are illuminated successively andsequentially in groups of P spots chosen from the N spots) and afrequency channel band.

Handing off is the change from a given state (radio frequency resourceallocation, connection station, satellite number) to a new state.Handing off can arise as a result of:

an improved elevation of the satellite as seen from the fixed cell,

interworking of the terminal with another connection station (movementof the terminal from one cell to another),

a more favorable satellite beam radio environment.

The decision to hand off depends on the availability of a satellitehaving a better elevation in the geographical area in which theterminals and the connection stations are located.

Handing off is based on the satellite ephemerides and the known radioenvironment of the mobile. It must be as fast as possible so that itdoes not affect calls already set up.

When handing off is effected a series of synchronization data is sent tore-establish a normal call. The synchronization procedure is identicalto that used in the terrestrial cellular network.

If resynchronization fails the terminal automatically returns to theradio frequency resources assigned to it before handing off and theoperation is repeated.

There are two feasible execution scenarios: progressive or globalhanding off of traffic.

If progressive handing off of traffic from one satellite to another isemployed, when a satellite has sufficient elevation on a satellite cellall new calls are set up via the new satellite. Then, as the quality ofservice provided by the first satellite deteriorates and the radioenvironment for a group of mobile terminals deteriorates, the decisionis taken to hand this traffic off to the new satellite. This procedurefavors progressive transfer of traffic from one satellite to the other.The radio environment of the terminals must be monitored continuously(monitoring beacon to be provided or direct communication with theterminals which then carry out series of measurements), as must thequality of service provided. FIG. 4 shows a connection station 20 withthree antennas 21, 22 and 23 and an associated satellite cell 24. Twoterminals 25 and 26 are connected to the station 20 via a firstsatellite 28. As the quality of service provided by the satellite 28deteriorates, the cell 24 is progressively reduced in size (24' then24"). The calling terminal 27 is connected to the station 20 directlyvia the second satellite 29. The decision to hand the terminals 25 and26 off from the first satellite (28) to the second satellite (29), asdefined above, are taken later (arrows 30 and 31).

If all of the traffic is handed off globally from one satellite to theother, when the new satellite has favorable elevation as seen from thefixed satellite cell and when all parameters of the mobile radioenvironment reach a specified limit, the instruction to hand off fromone satellite to the other is sent from the connection station to allthe terminals as shown by the arrow 32 in FIG. 5. This global procedureaffects all terminals in a given area. However, all the terminals mustsynchronize before continuing transmission and the allocations of radioresources to all the terminals must be taken into accountinstantaneously at the moment of handing off.

FIG. 6 explains preparation and execution of handing off.

The satellite having the best elevation for a given terrestrial area isselected from the ephemerides predictions for the constellation ofsatellites. This enables the connection station to acquire and track thenew satellite and to prepare for the allocation of radio frequencyresources.

The connection station decides to hand off and the instruction to handoff is routed via the link control subsystem. Execution of handing offentails the perfectly synchronized execution by the connection stationand the terminals of the following actions:

for the mobile: a change of transmit and receive time; this results in atranslation in time of the initial configuration,

for the connection station: handing off traffic to another antenna, onepointing to the new satellite.

Handing off uses the full range of GSM procedures without creating anynew procedure. In the system described here this procedure is governedby the connection station.

A particularly advantageous system utilizing the method described abovecomprises at least one satellite constellation which belongs to thegroup of prior art constellations known as Walker symmetricalconstellations. (See the article by J. G. Walker entitled "Continuouswhole earth coverage by circular-orbit satellites" in "Satellite systemsper mobile communications and surveillance", IEEE conference publication95, 1973). These constellations are symmetrical because the satellitesare regularly distributed in the same orbit and because of thedistribution and equal inclination of the orbital planes in space. Theyhave been chosen because they make it possible to minimize the number ofsatellites for a given coverage and are particularly efficient incovering a band of latitudes. A WALKER constellation is characterized byfive parameters:

The altitude, in this instance 1 389 km (for reasons of service life).

The inclination.

The set of three parameters T/P/F:

T is the total number of satellites,

P is the number of orbital planes,

F is a phase parameter which indicates the relative positions of thesatellites from one orbital plane to the next.

To optimize the coverage of inhabited areas, that is to say areasbetween the equator and 65° latitude (North or South), a WALKER (1 389km, 52°, 48/8/1) constellation is required. This constellation has theadvantage of enabling optimum coverage of the area, especially from thepoint of view of elevation, but like all constellations with a largenumber of satellites it has the drawback of requiring at least two yearsto set up. The system described here therefore uses two constellationswhich can be set up consecutively.

The constellations chosen are:

WALKER (1 389 km, 47°, 24/08/03) which covers correctly the CONUS andSouthern Europe (typically to the latitude of Lille) but which hasimportant coverage gaps below 30° latitude.

WALKER (1 389 km, 55°, 24/08/03) which covers the rest of the world andmakes it possible to optimize coverage, especially elevation, incountries between latitudes 10° and 60°.

Note that the first constellation of 24 satellites includes areas inwhich the minimal elevation is below that required. These areas aremobile and at low latitudes. The time for which no satellite is visibleat any given point is relatively short. For higher latitude areas theaverage elevation is clearly higher and the coverage is free of gaps.The defects are corrected by the launch of the second constellation.

Of course, the present invention has been described and shown by way ofpreferred example only and its component parts may be replaced byequivalent parts without departing from the scope of the invention. Theexplanations given above with reference to the first embodiment of themethod of the invention are naturally valid in respect of the secondembodiment: thus handing off from the first beam to the second may beprogressive or global.

There is claimed:
 1. A communication system, comprising:low orbiting satellites free of intersatellite links, including a first and a second satellite; a plurality of connection stations, each of which defines a respective fixed cell, and each of which provides a terrestrial communications network interface; a plurality of terminals, including (1) active terminals involved in a call, and (2) terminals on standby; and signal processing units, contained respectively in each of said satellites, in each of said plurality of terminals, and in each of said plurality of connection stations; wherein a respective position of each of said first and said second satellite is known at each of said plurality of said connection stations from respective satellite ephemerides of said first and said second satellite; wherein each of said plurality of terminals in a respective fixed cell communicates with a corresponding one of said plurality of connection stations via a respective satellite-terminal link with said first satellite, and further via a corresponding respective satellite-connection station link between said first satellite and said corresponding one of said plurality of connection stations; wherein each said respective satellite-terminal link and each said corresponding respective satellite-connection station link define a respective pair of links; and wherein said respective pair of links is handed off simultaneously and in real time from said first satellite to said second satellite after a handing-off decision by said corresponding one of said plurality of connection stations.
 2. The system as set forth in claim 1, wherein:said handing-off decision uses predetermined constraints and said ephemerides to ensure that synchronization time-delay, Doppler shift, and carrier frequency do not deteriorate beyond minimum performance limitations of said signal processor units; and wherein said predetermined constraints take into account a degree of uncertainty as to a position of each of said plurality of terminals in said respective fixed cell relative to said corresponding one of said plurality of connection stations.
 3. The communications system as set forth in claim 2, wherein:said handing-off decision is a single decision, is effected for all of said plurality of terminals in said respective fixed cell, and causes said respective pair of links for every one of said plurality of terminals in said respective fixed cell to be handed off.
 4. The communications system as set forth in claim 2, wherein:said handing-off decision is given effect for each respective pair of links according to the following rules:for each of said terminals on standby in said respective fixed cell, said handing-off decision causes all new terminal calls to be placed through said second satellite; and for each of said active terminals in said respective fixed cell, said handing-off decision is effected during said call when a quality of service of said call deteriorates to a predetermined threshold.
 5. The communications system as set forth in claim 3, wherein:each of said low orbiting satellites further comprises a multibeam antenna having a first and a second beam; wherein said respective pair of links is handed off simultaneously and in real time from said first beam to said second beam after a corresponding decision to hand off which is made by said corresponding one of said plurality of connection stations.
 6. The system as set forth in claim 5, wherein:said corresponding decision to hand off uses said predetermined constraints and said ephemerides to ensure that synchronization time-delay, Doppler shift, and carrier frequency do not deteriorate beyond said minimum performance limitations of said signal processor units; and wherein said predetermined constraints take into account said degree of uncertainty as to said position of each of said plurality of terminals in said respective fixed cell relative to said corresponding one of said plurality of connection stations.
 7. The communications system as set forth in claim 6, wherein:said corresponding decision to hand off is a single decision, is effected for all of said plurality of terminals in said respective fixed cell, and causes said respective pair of links for every one of said plurality of terminals in said respective fixed cell to be handed off from said first beam to said second beam.
 8. The communications system as set forth in claim 6, wherein:said corresponding decision to hand off is given effect for each said respective pair of links according to the following rules:for each of said terminals on standby in said respective fixed cell, said handing-off decision causes all new terminal calls to be placed through said second satellite beam; and for each of said active terminals in said respective fixed cell, said handing-off decision is effected during said call when said quality of service of said call deteriorates to said predetermined threshold. 